I note Android Auto updated last night and can now be used as a standalone app on the phone
I tried it this morning on way to work and have to say it is very simple but effective, the app opens once connected to chosen bluetooth device and will holds screen on (can be configured not to)
Its pretty much a near perfect and cost effective solution for cars provided you can plumb the audio through your stereo somehow
I'm using a Logitech [+]Drive windscreen mount, Kinivo BTC450 BT adaptor and the integration is near factory like, cost of around $100 + phone is a lot more palatable then cost and impossibility of swapping out headunit for an Android Auto device and best of all will always be up to date/futureproof